We propose to investigate the development of suppressor cell activity in mice bearing allogeneic, xenogeneic and syngeneic tumors. The suppressor cells inhibit proliferation of normal mouse spleen cells in mixed lymphocytes cultures (MLC). Tumors of various strains of origin and cell type, both recently induced and long established, will be tested for their ability to activate suppressor cells. Cytotoxic and suppressor T-cells will be separated on the basis of adsorption and possibly on the basis of XR and cortisone sensitivity. The metabolic requirements of suppressor cells and the nature of the factor(s) they produce will be investigated.